Hello Todd,
thank you very much for your interest in Weatherstock
The 4-members rule has to do with the concept of the whole event. Weatherstock is very large and "loud", like a huge festival in real life. We feel that smaller bands with less than four members just don't fit very well into the setting, scope and tone of the event.
Our suggestion would be to try to find some more friends to play together with you if that is possible and then reapply.
One more idea would be to apply for the Weatherstock Concert Series only. In the WCS we give Weatherstock bands the opportunity to present themselves to the audience for at least a full hour (often times they play much longer) in the two weeks before the big event. We already have some registrations of bands who only want to play at the WCS.